In general, a structural member such as a pipe or a turbine of a thermal power plant is used in a high temperature environment (about 600° C. at a maximum) and is applied with stress. Therefore, after a long-term operation, various kinds of damage such as creep damage unavoidably occur on a target portion of the structural member. Particularly, when creep damage progresses, creep voids and microcracks are generated in a grain boundary of a metal member. Finally, these creep voids and microcracks are connected with each other to generate large cracks, which may lead to breakage.
Therefore, in order to secure the reliability of a structure member, which is used at a high temperature, and to thereby stably operate a power plant or the like, it is important to accurately evaluate the damage of a target portion, to accurately calculate the lifetime, and to perform appropriate maintenance.
As a method of determining damage of a target portion of a structural member, an evaluation method focusing on a specific physical quantity is used. Such an evaluation method includes obtaining a relationship between a damage degree and a specific physical quantity in advance through an experiment, a simulation, or the like; and setting a damage evaluation index for evaluating a damage degree. By correlating this damage evaluation index with a measured value of the specific physical quantity at a target portion, a damage degree thereof is evaluated.
Patent Document 1 discloses a damage degree evaluation method including: measuring a temporal change rate in the number density of creep voids as a specific physical quantity; and correlating this temporal change rate in the number density of creep voids with a damage evaluation index to evaluate a damage degree thereof.
Patent Document 2 discloses a damage degree evaluation method including: measuring a maximum value of an occupancy ratio of creep voids in a grain boundary as a specific physical quantity; and correlating this maximum value of the occupancy ratio of creep voids in the grain boundary with a damage evaluation index to evaluate a damage degree thereof.